Summary of work: Studies have demonstrated that several cellular markers of oxidative stress are higher in cells from Alzheimer disease (AD) patients as compared to normal age- matched controls. These markers include oxidative damage to lipids, proteins and DNA in various tissues from AD patients. It has been proposed that AD cells may have a defect in the DNA repair processing of oxidative base lesion leading to accumulation of DNA damage. We have investigated the repair of oxidative base lesions using whole cell extracts from cultured AD lymphoblasts. DNA substrates containing both pyrimidine and purine lesions were obtained by treatment of plasmid with either gamma irradiation or fluorescent light (FL). Plasmid DNA containing primarily thymine glycol or 8-hydroxyguanine were prepared by damaging DNA with either OsO4 or methylene blue plus light ,respectively. The DNA substrates were purified free of strand breaks and were used in DNA repair synthesis assays. AD cells were proficient in repair of these substrates containing various oxidative base lesions. Extracts from AD cells repaired the plasmids damaged by gamma or FL-irradiation with equal efficiency as extracts from unaffected individuals. Our data indicate that the DNA damages resulting from the oxidative stresses used here are repaired efficiently in AD cells. Interestingly, the XPA cells were defective in the repair of certain oxidative DNA base modifications. These lesions may thus be repaired by nucleotide excision repair rather than base excision repair. It is possible that the repair of specific oxidative base damages, such as that seen for thymine glycol and 8-hydroxyguanine, may be upregulated in AD due to chronic oxidative stress which has been previously implicated in this disease. There have been reports of unusual accummulation of oxidative DNA damage in mitochondrial DNA from patients with AD, and we have recently developed novel techniques to study DNA repair in these organelles. We have used a mouse model system for AD, transgenic mice expressing the amyloid precursor protein 1 (APP1) gene. APP1 is the precursor of the beta amyloid protein, which accumulates in the brain of AD patients and have been shown to induce accumulation of 8OHdG in mitochondrial DNA. Our results show that liver mitochondria from APP1 mice have slightly higher repair of 8OHdG than wt animals. However, when the animals were kept in a folic acid deficient diet, mitochondrial extracts from APP1 mice show significantly higher 8OHdG incision activity that wt. Since the folic acid deficient diet is associated with higher levels of homocysteine and oxidative stress, these results are consistent with an upregulation of base excision repair in mitochondria by chronic oxidative stress. In conclusion, our results suggest that accumulation of oxidative base damage in the mtDNA may be involved in the ethiology of AD.